1. Field of the Invention
The present invention relates, in general, to power generation and conversion, to cryogenic systems, and to improvements in heat engines and systems, and more particularly, to energy conversion systems and methods that utilizes a working fluid, such as a biatomic gas, that is recirculated within a high pressure tank that contains a compressor and is exposed to a heat exchange surface to enhance existing heat engine efficiencies and that further utilizes a cold reservoir to capture additional energy from the working fluid.
2. Relevant Background
Modem society has an insatiable and growing thirst for energy and for devices and systems that consume large quantities of energy. Presently, the largest sources of energy are non-renewable including the fossil fuels of coal, oil, and gas. Renewable energy sources are only a small portion of the global energy supply and include wind, solar, and geothermal sources. Energy sources are generally converted by conversion systems using heat engines and other devices into other forms of energy such as thermal energy (or heat) and mechanical energy. It is estimated that in the not too distant future non-renewable energy sources will become depleted or that the costs associated with converting these sources to heat and other useful energy will significantly increase causing many of these sources to be inaccessible to large parts of the population. Hence, there is an ongoing societal need for more efficient methods and systems for converting energy from non-renewable and renewable energy sources into clean, useful energy.
Common energy conversion systems employ heat engines to convert heat energy from renewable or non-renewable energy sources to mechanical energy. The examples of heat engines are numerous including steam engines, steam and gas turbines, spark-ignition and diesel engines, or external combustion and the Stirling engine. Each of these heat engines or systems can be used to provide the motive power or mechanical energy for transportation, for operating machinery, for producing electricity, and for other uses. Heat engines typically operate in a cycle of repeated sequences of heating and pressurizing a working fluid, performing mechanical work, and rejecting unused or waste heat. At the beginning of each cycle, energy in the form of heat and/or pressure is added to the working fluid forcing it to expand under high pressure so that the fluid performs mechanical work. In this manner, the thermal energy contained in the pressurized fluid is converted to kinetic energy. The fluid then loses pressure, and after unused energy in the form of heat is rejected, the fluid is reheated or recompressed to restore it to high pressure.
Unfortunately, existing heat engines do not convert all the input energy to useful mechanical energy in the same cycle as generally some amount often in the form of heat is not available or utilized for the immediate performance of mechanical work. The fraction of thermal energy that is converted to net mechanical work is called the thermal efficiency of the heat engine. The maximum possible efficiency of a heat engine is that of a hypothetical or ideal cycle, called the Camot Cycle (based on absolute zero as the starting point). Existing heat engines generally operate on much less efficient cycles, such as the Otto, Diesel, Brayton, or Stirling Cycles, with the highest thermal efficiency achieved when the input temperature is as high as possible and the sink temperature is as low as possible. The xe2x80x9cwastexe2x80x9d or rejected heat is sometimes used for other purposes, including heating a different working fluid, which operates a different heat-engine cycle or simply for space heating but most often the rejected heat is released to the environment. Another common efficiency problem is that when compressors are used to compress incoming air or working fluid and are driven by a shaft driven by the device creating the mechanical power, e.g., a turbine using the Brayton cycle, and the compressor consumes a large portion of the created shaft power, e.g., up to two-thirds of the power.
Hence, there remains a need for improved devices and techniques for converting energy in a working fluid with increased efficiencies. Preferably, such devices and techniques are configured to utilize well-known or even off-the-shelf devices, including heat engines or heat engine components, in new arrangements that enable the capture or collection of a larger percentage of heat from a working fluid and that, in some cases, provide energy in a number of forms including electricity, heat, refrigeration or cooling, and the like.
The present invention addresses the above problems by providing energy conversion systems and corresponding methods that are adapted to make power and cooling (e.g., cryogenic and coolant flow for refrigeration and heat transfer). The energy conversion systems of the invention generally include an artificially maintained cold reservoir or loop that is retained out of equilibrium with the surrounding environment or ambient fluids (e.g., air, water, and the like). An expander heat engine is included in the system to produce mechanical power from the expansion of a working fluid, such as a binary gas. The expander is thermally and pressure isolated and receives the relatively high-pressure working fluid that has been heated by a heat exchanger to a temperature higher than the thermally isolated expander. Some of the power generated by the expander is, at least in some embodiments, used to perform forced rarefaction of the working fluid and to power a cooling cycle in which condensated working fluid or condensate from the expander and/or rarefaction is pumped to a cold reservoir to export additional heat obtained by the working fluid in the thermally isolated heat exchanger or heat transfer zone. The amount of energy or heat diverted from power production for the purpose of lowering or creating the cold reservoir is preferably at least equal to the friction of the expander section of the heat engine as this energy allows the system to continue operation without reaching equilibrium by eventually running down.
Generally, the engine is selected to be a relatively large volume engine relative to the engines size and/or crank shaft. Typically, the system can be fabricated from common industrial materials and components such as those used for internal combustion engines and bearings. One embodiment of the system uses off-the-shelf components including pumps, engines, and compressors that are built to tolerances and with materials selected to operate within the pressure and temperature ranges of the system, i.e., very cold temperature ranges compared with internal combustion engines. For example, a swash plate piston motor used in hydraulics and air conditioning can be used as an expander, which provides a high ratio of working area to power train linkage and weight. The components are tuned for their designed operating temperatures including calculating any changes for differential shrinkage of rings or bearings, and a lubricant used for cryogenic pumps can be utilized for the expander and related components with beryllium copper and materials chosen for cryogenic systems used in the system of the invention (such as for springs for seals or valves).
After expansion and cooling, a compressor is provided in the system to recompress the now less energetic working gas. The compressor is typically positioned within the same pressure vessel as the expander with a heated or working portion of the compressor exposed to the cold or low temperature side of the heat exchanger to provide the heat of compression to the working fluid rather than rejecting it to the environment. The compressor injects or discharges the compressed working gas to the cold side of the heat exchanger where the gas absorbs heat from an input hot or energy-source fluid such as ambient air, compressed gas (heated or unheated by combustion processes or other heat sources), water or other fluids (e.g., ocean or other large bodies of water), fluids heated by geothermal sources, rejected hot gases and fluids (e.g., from internal combustion engines, manufacturing processes, or any other heat generating process), and the like. According to the invention, a load, e.g., a mechanical load, an electrical load such as a power grid, a device performing work by pumping fluid, or any other useful device or system for performing work, is provided outside the thermal barrier or vessel containing the expander and the compressor, to allow exportation of the power converted within the vessel. In most embodiments, a shaft-driven motor generator device is provided for converting the mechanical energy created by the expander, which is linked to the generator shaft, so that electricity can readily be exported from the thermal barrier. In one embodiment, the amount of energy exported is selected to be about equal to frictional losses in the expander and generally not less than about one third of the total energy produced by the expander. The motor generator device is operated in a motor mode to start the expander (and, generally, the compressor) to start the energy conversion system.
The energy conversion system preferably is operated at a relatively steady speed or rate after it has been started and continues to run as long as a temperature difference is maintained between the input or energy-source fluid and the working fluid on the cold side of the heat exchanger. A controller or regulator is provided to control the ongoing operation of the system by setting the circulation of working fluid, controlling mass flow of the energy-rich fluid in the cold or working side of the heat exchanger, the size of the outside load, the supply of working fluid (e.g., if replenishing of working fluid is required due to condensate collection or other reasons), and valve and/or piston timing. Note, that generally it is more important to provide a useful rate of working gas flow through the system (e.g., through the channel or flow maze created in the system) to cause heat to be transferred as desired and to achieve proper energy or heat flow in the system than to attempt to provide high or absolute insulation of the expander or isolation vessel. A large portion of the heat energy converted to mechanical energy and other forms of useful energy such as electricity is transferred outside of the thermal barrier or low-pressure expander vessel. The remainder of the work is used inside the system to recompress the working gas, to pump condensed working fluid, to maintain the artificial cold reservoir in or associated with the expander heat engine, and to overcome mechanical component or other inefficiencies (such as friction), but, significantly, due to the configuration of the system and the positioning of the components, the remainder of the work or energy is xe2x80x9crejectedxe2x80x9d within the system rather than to the surrounding environment so that this energy is recirculated and available for conversion again and is not wasted.
More particularly, an energy conversion device is provided for converting heat to mechanical energy, i.e., converting low grade energy to useful high grade energy. The apparatus includes a heat exchanger vessel with walls adapted for absorbing heat from an input ambient or energy-rich fluid and transferring the absorbed heat to a working fluid within an interior chamber of the vessel. The heat exchanger vessel is typically a pressure vessel to resist relatively high pressures developed within the interior chamber. An isolation vessel is positioned within the interior chamber of the heat exchanger vessel, with a gas flow loop defined between the two vessels through which a compressed high pressure gas flows (i.e., a cold side of a heat exchanger). The isolation vessel includes an outer wall that is insulated to thermally isolate an interior volume to create a desired temperature differential and is pressure resistant and sealed to allow low pressures relative to those in the interior chamber of the heat exchanger vessel to be maintained. A compressor is positioned within the isolation vessel to receive expanded, cool work gas, to compress the gas, to discharge the gas into the gas flow loop, and to discharge or reject the heat of compression within the interior volume or into the gas flow loop.
An expander is also positioned within the low pressure, low temperature isolation vessel with an inlet in the working gas loop to receive the compressed, heated working gas. The expander includes an expansion device for converting work performed by the expanding gas into a mechanical energy and to discharge expanded, cooled gas into the interior volume of the isolation vessel. The expander preferably includes a device for rarefying the expanded gas prior to discharge to further cool the gas and create a lower bottom temperature of the cold reservoir. The expander typically uses the mechanical energy to rotate a drive shaft, which is linked in some cases to the compressor and to a motor/generator set. The motor/generator set is utilized to generate electricity from the rotation of the drive shaft that is exported from the isolation vessel to an external load. A cooling system can be provided to further chill the expanded and rarefied working gas to condense a portion of the working gas, and the condensate can be collected for export from the system, i.e., for use as-is such as for liquid nitrogen or for use in a cryogenic loop in which case the heated condensate can be returned to the device in the gas flow loop to add additional energy to the device.